Worldwide, there are ever-growing demands for electricity due to increasing populations, technology advancements requiring the use of electricity, and the proliferation of such technology to more and more countries around the world. At the same time, there is an increasing push to harness reusable sources of energy to help meet these increasing electricity demands and offset and/or replace traditional carbon-based generators which continue to deplete natural resources around the world.
Many solutions have been developed to collect and take advantage of reusable sources of energy, such as solar cells, solar mirror arrays, and wind turbines. Solar cells produce direct current energy from sunlight using semiconductor technology. Solar mirror arrays focus sunlight on a receiver pipe containing a heat transfer fluid which absorbs the sun's radiant heat energy. This heated transfer fluid is then pumped to a turbine which heats water to produce steam, thereby driving the turbine and generating electricity. Wind turbines use one or more airfoils to transfer wind energy into rotational energy which spins a rotor coupled to an electric generator, thereby producing electricity when the wind is blowing. All three solutions produce electricity when their associated reusable power source (sun or wind) is available, and many communities have benefited from these clean and reusable forms of power.
Unfortunately, when the sun or wind is not available, such solutions are not producing any power. In the case of solar solutions, non-reusable energy solutions are often turned-to overnight. Similar issues arise for wind turbines during calm weather. Therefore, some form of energy storage is needed to store excess energy from the reusable power sources during power generation times to support energy demands when the reusable power source is unavailable or unable to meet peak demands for energy.
Solar mirror arrays generate and transfer heat as an inherent part of their operation. Solar cells and wind turbines which typically generate electricity can also selectively be used to drive heaters to generate heat and/or transfer heat from windings to a heat transfer fluid. Several solutions have been developed to store heat from these renewable energy sources for use in non-energy-generating times.
FIG. 1 illustrates a two-tank direct energy storage system. Heat transfer fluid is heated by mirrors in a solar field 30 and stored in a hot oil tank 32. The heat transfer fluid is then pumped through a steam generator 34 as needed to generate steam and power a turbine 36 to meet energy demands. Even if the solar field 30 is not producing newly heated heat transfer fluid for the hot oil tank 32, the hot oil tank 32 has a certain capacity to provide stored hot transfer fluid to the steam generator 34 for power generation. After passing though the steam generator 34, the cooled heat transfer fluid is then pumped into and stored in a cold oil tank 38. When the solar field 30 is active, cooled heat transfer fluid is pumped from the cold oil tank 38, through the solar field to be heated-up, and back to the hot oil tank 32 where the process can begin again. While the two-tank direct energy storage system of FIG. 1 helps to store energy for non-generation times, it is unfortunately complex, requires two expensive tanks, and is limited in the amount energy it can store due to limitations in the heat storage capacity of the heat transfer fluid.
FIG. 2 illustrates a two-tank indirect energy storage system. Relatively cold molten salt is pumped from a cold salt tank 40 out to a heat exchanger 42 where it is heated by proximity to counter-current running hot heat transfer fluid from the solar field 44. The newly-heated molten salt is then pumped from the heat exchanger 42 into a hot salt tank 46 where it is stored until needed. When energy needs to be reclaimed from the hot salt tank 46, the hot molten salt is pumped out of the hot salt tank 46 and to a turbine system 48 whereby the heat from the hot molten salt is used to generate steam to drive the turbine system 48. Relatively cold molten salt exits the turbine system 48 and is pumped back into the cold salt tank 40. Alternatively, the hot molten salt from the hot salt tank 46 may be pumped out of the hot salt tank 46 and back through the heat exchanger 42 to heat the heat transfer fluid from the solar field 44 before being pumped back into the cold salt tank 40. In this alternate setup, the reheated heat transfer fluid would then be pumped through the turbine system before being recirculated to the solar field. Taking advantage of the heat storage capacities of salt in this indirect two-tank system, more energy may be stored than in the direct system. Unfortunately, this system still requires two expensive tanks. Furthermore, the system of FIG. 2 will be subjected-to complexities and issues arising from the need to pump and transport molten salt. The system may have the need to keep the salt molten at all times and therefore may require the addition of heaters not powered by the solar field. If the salt is allowed to solidify within the transport pipes, the natural expansion of the salt when transitioning from the solid state back to the liquid state may cause stress cracks in the pipes. Furthermore, if the salt is allowed to solidify, the system may take an undesirable amount of time to come on-line as it waits for the salt to liquefy to become pumpable. Corrosion is also an issue when pumping molten salt.
FIG. 3 illustrates a single-tank thermocline energy storage system. The thermocline tank 50 holds a hot molten salt on the top of the tank 50 and a relatively cool molten salt in the bottom of the tank 50. When the solar field 52 is active, a hot heat transfer fluid is pumped from the solar field to a heat exchanger 54. The relatively cool molten salt is pumped out of the bottom of the thermocline tank 50 out to the heat exchanger 54 where it is heated by proximity to the hot heat transfer fluid from the solar field. The heated molten salt is then returned to the top of the thermocline tank 50. When the solar field 52 is not active, the flow to and from the thermocline tank 50 is reversed. Heated molten salt is pumped out of the top of the thermocline tank 50 to the heat exchanger 54, where it transfers its heat to the heat transfer fluid. The heat transfer fluid is pumped to a turbine system 56 for generating electricity. The molten salt which gave up some of its heat in the heat exchanger 54 is then returned to the bottom of the thermocline tank 50. While this system takes advantage of a vertical temperature gradient within the thermocline tank to move down to a single tank, the tank itself may still be expensive when properly sized for industrial and/or community demands, and the system continues to have the corrosion and solidification concerns mentioned above when pumping molten salt.
Therefore, there is a need for a thermal energy storage system which can take advantage of the high energy storage capacities of phase change media, such as salts, while avoiding corrosion and solidification issues in an inexpensive, scalable, easy-to-construct, control, and maintain fashion.